Graft copolymers of polypropylene have been of interest for some time because they are capable of possessing some properties of the grafted polymer (produced by the homo- or copolymerization of the monomer or monomers, respectively) as well as of the polypropylene backbone. It has been suggested, for example, that some of these graft copolymers be used as compatibilizers for normally immiscible polymer systems and as components of gum plastic compositions.
The preparation of graft copolymers by creating active sites on the main polymer chain or backbone, and initiating graft polymerization of a polymerizable monomer at these sites, is well-known. Procedures which have been used for introducing such active sites into the polymer chain have included treatment with organic chemical compounds capable of generating free radicals, and irradiation. In the chemical method, an organic chemical compound capable of generating free radicals, such as a peroxide or azo compound, is decomposed in the presence of the backbone polymer with the formation of free radicals, which form the active grafting sites on the polymer and initiate the polymerization of the monomer at these sites.
Of the various techniques which have been employed for preparing graft copolymers of polyolefins by the chemical method of free radical generation, the bulk technique, in which the polymer particles are contacted directly with the initiator and monomer, without the intervention of a liquid suspending medium or a solvent, is advantageous in terms of simplicity of execution and the avoidance of side-reactions caused by the presence of certain solvents or suspending media, such as water. However, regardless of the physical state of the polymer to be grafted, the grafting process is subject to problems such as degradation of the polyolefin, possibly leading to a graft copolymer having an undesirably high melt flow rate, and excessive formation of the homopolymer of the grafting monomer at the expense of the formation of the polyolefin graft copolymer.
U.S. Pat. No. 4,595,726 discloses graft copolymers of 3-100%, preferably 3-30%, by weight of an alkyl methacrylate moiety grafted onto a polypropylene backbone. The graft copolymers, useful as adhesives in polypropylene laminates, are prepared at a temperature below the softening point of polypropylene by a solvent-free reaction, reportedly vapor-phase, between polypropylene and the methacrylate monomer in the presence of a free-radical-forming catalyst. A preferred initiator is tert-butyl perbenzoate, stated as having a 15-minute half-life at 135.degree. C., and reactor temperatures of 135.degree. C. and 140.degree. C. are disclosed. Degradation of the polypropylene chain due to the reaction conditions employed is reported. Immediately after the peroxide is added to the polypropylene, the monomer is added over a time period which is fixed by the half-life of the peroxide initiator (i.e., 1-2 half-lives). In other words, according to the teachings of U.S. Pat. No. 4,595,726, for a given initiator half-life, it is necessary to employ a higher rate of addition of the monomer as the total amount of monomer to be added increases.
The preparation of "graft-type" copolymers by dissolving an organic peroxide in a monomer and adding the solution to free-flowing particles of the base polymer, particularly polyvinyl chloride, is described in U.S. Pat. No. 3,240,843. The "graft-type" products are described as having monomeric, as opposed to polymeric, branches attached to the polymer backbone. Homopolymerization of the monomer also is mentioned. To avoid particle agglomeration, the amount of monomer added cannot exceed the maximum absorbable by the polymer particles. In the case of polypropylene charged into a reactor with a solution containing styrene, butadiene, acrylonitrile, and benzoyl peroxide, the total amount of monomers added is only 9% of the amount of polypropylene charged.